Molecular beam epitaxial growth of III-V semiconductor ... - KOBRA

Experimental Growth and Characterization Techniques
4.2 Molecular Beam Epitaxy Technique
Molecular beam epitaxy is a versatile epitaxy technique with the capacity for
monolayer-scale control, due to the very low growth rates which can be achieved
by the evaporation cells. The growth rate of typically 1µm/h (1 monolayer/s) is
low enough that surface migration of impinging species on the growing surface is
ensured. What distinguishes MBE from other vacuum deposition techniques is
the signicantly more precise control of the beam uxes and growth conditions.
The capabilities of realizing well-controlled abrupt interfaces, doping proles and
alloy heterojunctions oer many opportunities to implement device structures
which have not been practical or realizable in the past. Simple mechanical shutters
in front of the beam sources are used to interrupt the beam uxes, i.e., to
start and to stop the deposition and doping. Changes in composition and doping
can thus be abrupt on an atomic scale [33].
In SMBE, thin lms crystallize via reactions between thermally evaporated
molecular or atomic beams of the constituent elements and a substrate surface
which is maintained at an elevated temperature in ultra-high vacuum. Because of
UHV deposition, MBE growth is carried out under conditions far from thermodynamic
equilibrium and is governed mainly by the kinetics of the surface processes
occurring when the impinging beams react with outermost atomic layers of the
substrate crystal. The composition of the grown epilayer and its doping level depend
on the relative arrival rates of the constituent elements and dopants, which
in turn depend on the evaporation rates of the elements from solid or liquid phase
[33].
In this thesis an MBE system is used, which is composed by three main
chambers with dierent vacuum levels; exit/entry chamber (E/E chamber) or
sometimes called as load and unload chamber with a background pressure in
the order of 10 −8 T orr. The buer chamber serve as preparation chamber before
the growth, which contains also high temperature desorption (HTD) station
for initial thermal desorption of the substrate and with vacuum level up
to 10 −9 T orr and nally the growth chamber (growth reactor) as it is shown in
Fig. 4.1 equipped with high temperature manipulator for substrate heating, holding
and rotation. The pressure in an idling MBE growth chamber is maintained
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